1. Field of the Invention
This invention relates to paper sizing compositions containing a) sizing agent, b) dispersant system, and c) inorganic salt, and also to methods of sizing paper using such compositions, and to products made using such sizing compositions.
2. Background of the Invention
Cellulose-reactive sizing agents, such as alkyl ketene dimers (AKDs), are widely used in the paper making industry as components in sizing dispersion formulations. The usefulness of such agents derives from their ability to bond directly to the hydroxyl groups on the cellulose component of the paper. Frequently, in these sizing formulations, the sizes, e.g., AKDs, are combined with dispersant systems which include cationic starch and/or sodium lignosulfonate. Examples of such dispersions can be found in U.S. Pat. No. 4,861,376, to Edwards, and U.S. Pat. No. 3,223,544, to Savina, the disclosures of which are hereby incorporated by reference.
The usefulness of inorganic salts such as alum, or aluminum containing salts, in paper making processes has also been known for some time. Practically, these salts greatly improve machine productivity by enhancing dewatering. As a general rule, the higher the concentration of the salts, the better the dewatering capacity.
The combination of organic sizing agents such as AKD dispersions and inorganic salts such as alum is thus a valuable combination, providing both sizing and dewatering capacity. Examples of such combinations are presented in U.S. Pat. No. 5,145,522 to Nakagawa et al. and U.S. Pat. No. 5,627,224, to Lyrmalm et al., the disclosures of which are hereby incorporated by reference. The problem encountered by this combination, however, is that organic sizing agents tend to be quite incompatible with inorganic salts, separating out if the concentration of the inorganic salts becomes too high in a liquid phase of the organic sizing dispersion.
There is, therefore, a need in the art for a dispersant system which is compatible with inorganic salts and organic sizing agents. A number of dispersant systems have been described in the literature. For example, in U.S. Pat. No. 5,627,224, an amphoteric polymer is used as the sole dispersant to incorporate polyaluminum compounds in dispersions of AKD. However, achieving good stability in compositions containing organic sizes and inorganic salts has been elusive. Thus, there is a need in the art for compositions which solve the aforementioned problems.